1. Field of the Invention
The present invention relates to automatic control systems for vehicle engines. In particular, the invention relates to a method for controlling the closing phase of a clutch in an automated automobile transmission system.
2. Description of the Prior Art
Automated transmission systems, or automated manual transmission (AMT) systems, are an intermediate solution between conventional manual transmissions and automatic transmissions. Unlike the latter, characterized by complex mechanical components such as torque converters and epicyclic trains, automated transmissions use devices (clutch, gearbox) having conventional mechanics controlled not by the driver but by one or more on-board control unit(s). FIG. 1 shows a generic diagram of an automated transmission. FIG. 2 shows a conceptual diagram of an automated transmission.
Any automated transmission control system, having on-board one or more ECUs (Electronic Control Units), has to generate set points for:
(electric, pneumatic or hydraulic) control of the position of the clutch (clutch actuator);
(electric, pneumatic or hydraulic) control of the gears of the gearbox (gearbox actuator(s)), and expressing the gear change selected by the driver.
Thus, in an automated transmission, it is the ECUs dedicated to transmission control that allow responding to the driver's requests (standing start and gear shift), through the aforementioned actuators, in collaboration with the engine control ECU that determines the torque produced by the engine.
Standing start is the maneuver of setting a stationary vehicle in motion by transmitting an increasingly large fraction of the engine torque to the primary transmission shaft, therefore to the wheels, through progressive closing of the clutch. The clutch (progressively, then fully) couples the engine flywheel to the primary shaft, thus transmitting the torque produced in the engine (minus the friction of the engine crankshaft assembly) to the primary shaft. In this scheme, the clutch can therefore be:
completely open (disengaged, out of gear), with a zero torque transmitted to the primary shaft,
completely closed (engaged, geared), with the engine torque fully transmitted to the primary shaft,
sliding, closing or opening. It is in the closure sliding phase that progressive transmission of the engine torque to the primary shaft takes place.
In the case of a conventional manual transmission, the driver achieves progressive closing of the clutch by adjusting simultaneously the pressure exerted on the clutch pedal and the pressure exerted on the accelerator pedal. His or her experience as a driver will determine the successful outcome of the maneuver or its failure (engine stalling, over-revving, strong oscillations). In the case of an automated transmission, the clutch pedal is absent and it is the transmission control system that adjusts the action of the clutch and coordinates it with the engine torque production so as to ensure smooth progress of the standing start maneuver.
Progressive closing of the clutch and its coordination with the engine torque production thus is the key phase of the automated transmission control. To understand the operation of the powertrain in this phase, the torque transmitted by the clutch (engine torque fraction) has to be considered as a negative torque on the crankshaft side (thus decreasing the net torque provided by the engine) and as a positive torque on the transmission side downstream from the clutch, minus the transient and static losses due to elasticities, friction and the efficiency of each mechanical element. It is this torque that, geared down by the gear ratio, is transmitted to the wheels. The state of the powertrain is then defined by variables measured upstream and downstream from the clutch. Typically, the engine speed upstream, is always available on a vehicle, plus (at least) a speed measurement downstream from the clutch which is primary shaft speed, secondary shaft speed or wheel speed. These variables are not systematically measured on a standard vehicle, but they have to be measured for a vehicle equipped with an automated transmission.
In order to fulfill the main two functions of an automated transmission system, standing start and gear shift, the control systems of standard vehicles generally calculate the set point of the clutch actuator from pre-filled charts (mapping) as a function of the torque required by the driver (accelerator pedal position), the engine speed, the primary shaft speed (or other speed on the transmission side) and other parameters such as the gear ratio (in the case of gear shifting).
Regarding the engine control, the engine speed must be ensured to remain compatible with the maneuver being considered, despite the negative torque acting on the crankshaft (the engine control therefore has to increase the engine torque accordingly). From a systemic point of view, this type of control structure is but a particular case of the general structure of FIG. 3. This diagram shows that, in the control of the clutch sliding phase, there are two “levers” which are the engine torque that acts only upon the crankshaft and the torque transmitted by the clutch, which acts both on the crankshaft (as a negative torque) and on the downstream of the transmission up to the wheels. There are, for the control system, and using the Automatic terminology, two actuators that act at the input of the powertrain system. In order to know the state of the system and to act accordingly, it is necessary to measure at least two outputs, the engine speed and one of the clutch downstream speed which, for example, may be the primary shaft speed. The inputs of the powertrain system, that is the engine and clutch torques, are never measured in a standard vehicle and are only assessed, with much imprecision.
This mapping control, typical of standard vehicles, does not allow readily translating the specifications to be met during clutch closing which are compliance with the driver's requests, comfort and maintenance of powertrain smooth running. Besides, a long calibration time to fill in the maps.
In order to do without methods based on mapping, there are known solutions based on control laws inspired by the Automatic principles. These laws are no longer based only on charts that have been previously filled in. These solutions are based on algorithms that calculate the input data to be sent to the powertrain system, engine torque and clutch torque, from measurements of the state of this system (typically engine speed and primary shaft speed), designed using the feedback principle.
For these feedback control laws to be usable within the context of the engine control of a vehicle, they must allow meeting a number of specifications which respect constraints guaranteeing smooth running of the thermal engine, constraints guaranteeing comfort upon clutch engagement (no oscillation), and the driver's will.
Now, these techniques cannot explicitly manage these constraints to ensure that one or more variables of the system to be controlled (inputs, outputs or state), or their derivatives, do not exceed certain limits set as specifications.